The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Shock absorbers are used in conjunction with automotive suspension systems to absorb unwanted vibrations which occur during driving. Shock absorbers are generally connected between the sprung portion (body) and the unsprung portion (wheels) of the automobile. A piston is located within a working chamber defined by a pressure tube of the shock absorber, with the piston being connected to the sprung portion of the automobile through a piston rod. The pressure tube is connected to the unsprung portion of the vehicle by one of the methods known in the art. Because the piston is able, through valving, to limit the flow of damping fluid between opposite sides of the piston when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force which damps the unwanted vibration which would otherwise be transmitted between the unsprung portion and the sprung portion of the automobile.
Shock absorbers have been developed to provide different damping characteristics depending upon the speed or acceleration of the piston within the pressure tube. Because of the exponential relation between the pressure drop and flow rate, it is difficult to obtain a damping force at relatively low piston velocities, particularly at velocities near zero. Low speed damping force is important to vehicle handling since most vehicle handling events are controlled by low speed vehicle body velocities. It is also important to control damping force over the broad range of pressures generated across the piston as the piston velocity increases.
Various prior art systems for tuning shock absorbers during low speed movement of the piston have integrated a bleed flow of the hydraulic fluid. In piston valving with this integrated bleed flow, various alternatives for incorporating the bleed flow are possible. A fixed low speed bleed orifice can be created by utilizing orifice notches positioned either on the flexible disc adjacent the sealing land or by utilizing orifice notches directly in the sealing land itself. Another alternative is to incorporate straight bleed holes that end in a horse-shoe or circular shaped valve area which is then closed using a low speed valve disc. The designs with cross-drilled holes have been found to have a slightly better performance on hydraulic flow noise but a cross-drilled set of holes does not lend itself to the economics of mass production. In addition, using cross-drilled holes requires that a different piston will have to be designed for different applications since the number and diameter of the bleed holes are a tuning parameter. This fact also increases the complexities associated with mass production of shock absorbers.